Connection (or ligation in biochemistry) of two or more substrates or immobilization of various compounds are often achieved with the help of “click chemistry,” which describes a set of bimolecular reactions that are modular, wide in scope, high yielding, create only inoffensive by-products, are stereospecific, simple to perform and that require benign or easily removed solvent. Although meeting all of the above requirements is difficult to achieve, several processes have been identified as coming very close to the ideal “click reaction.” Among them are 1,3 dipolar and Diels-Alder cycloadditions, nucleophilic ring opening, non-aldol carbonyl chemistry, and additions to carbon-carbon multiple bonds. Cu(I) catalyzed versions of the Huisgen acetylene-azide cycloaddition, also known as azide click reaction, became the gold standard of click chemistry and have been applied in fields ranging from material science to chemical biology and drug development. However, the use of cytotoxic Cu (I) catalysts has largely precluded application of this click reaction in living systems. Recently discovered catalyst-free 1,3-dipolar cycloaddition of azides to cyclooctynes and dibenzocyclooctynes offers a bio-compatible version of the azide click reaction.
“Click” methods based on a Diels-Alder cycloaddition are also gaining popularity due to the fact that this reaction does not require catalysts, can proceed in high yield under physiological conditions, and does not produce any by-products. However, Diels-Alder cycloaddition reactions are often slow and require either thermal activation or the use of chemical promoters for the in situ generation of highly reactive dienes.
Thus, there remains a need for catalyst-free ligation methods for connection or immobilization of various compounds.